just reinstall all fonts i guess you caused a font conflict nothing else as there are different helveticas around and fonts ar not just a bunch of letters , you could even prevent booting with the wrong fonts
here is the whole article about fonts you can read it if you find the time as its a long article covering all osx versions really
http://www.jklstudios.com/misc/osxfonts.html
but i extracted the main bit you need
First, download
Pacifist. Insert the first OS installation CD or DVD. Start the utility and click on the "Open Mac OS X Install Packages" button. Keep feeding the application the disc of your Leopard/snow leopard Install disks as it asks for them. If you have a set of DVDs that came with your Mac, the first disk should be the only one you need. After Pacifist has finished reading all of the packages, select the package that has the fonts you want installed and click the Install button on Pacifist's menu. Only that package will be installed to its default location. For a complete set of fonts, reinstall both the /System/Library/Fonts/ and /Library/Fonts/ folders. All fonts you install from Pacifist will overwrite any identical fonts on your hard drive in the default locations, replacing any damaged fonts in the process. If you have copied or moved any of the default fonts to another location, they will still be there and will need to be deleted manually. All in all, this method is much quicker and easier than doing an entire Archive and Install to get your fonts back where they belong.
Font locations for the Leopard and Snow Leopard retail disks:
For the System folder fonts, expand Contents of OSInstall.mpkg > Contents of EssentialSystemSoftware > Contents of EssentialSystemSoftwareGroup > Contents of BaseSystem.pkg > System > Library > Fonts and highlight the folder "Fonts" by clicking on it once. Click "Install" at the upper left of the Pacifist window.
For the Library folder fonts, expand Contents of OSInstall.mpkg > Contents of EssentialSystemSoftware > Contents of EssentialSystemSoftwareGroup > Contents of Essentials.pkg > Library > Fonts and highlight the folder "Fonts" by clicking on it once. Click "Install" at the upper left of the Pacifist window.
The Additional Fonts are in two separate locations. Expand Contents of OSInstall.mpkg > Contents of AdditionalFonts.pkg > Library > Fonts, or Contents of OSInstall.mpkg > Contents of AdditionalFonts.pkg > System > Library > Fonts, depending on the set of additional fonts you want to restore.
good luck and dont do it again
